PLANT ACYL ACP THIOESTERASE SEQUENCES

Abstract: This invention relates to plant thioesterases, specifically plant acyl-ACP thioesterases having substantial activity on palmitoyl-ACP substrates. DNA constructs useful for the expression of a plant palmitoyl-ACP thioesterase in a plant seed cell are described. Such constructs will contain a DNA sequence encoding the plant palmitoyl-ACP thioesterase of interest under the control of regulatory elements capable of preferentially directing the expression of the plant palmitoyl-ACP thioesterase in seed tissue, as compared with other plant tissues, when such a construct is expressed in a transgenic plant. This invention also relates to methods of using a DNA sequence encoding a plant palmitoyl-ACP thioesterase for the modification of the proportion of free fatty acids produced in a plant seed cell. Plant palmitoyl-ACP thioesterase sequences exemplified herein include Cuphea, leek, mango and elm. Transgenic plants having increased levels of C16:0 fatty acids in their seeds as the result of expression of these palmitoyl-ACP thioesterase sequences are also provided.

Description: PLANT ACYL ACP THIOESTERASE SEQUENCES * INTRODUCTION 5 Field of the Invention This invention relates to the application of genetic engineering techniques to plants. More specifically, the invention relates to plant acyl-ACP thioesterase sequences and methods for the use of such sequences. 10 BACKGROUND Fatty acids are organic acids having a hydrocarbon chain of from about 4 to 24 carbons. Many different kinds of fatty acids are known which differ from each other in chain length, and in the presence, number and position of 15 double bonds. In cells, f tty acids typically exist in covalently bound forms, the carboxyl portion being referred to as a fatty acyl group. The chain length and degree of saturation of these molecules is often depicted by the formula CX:Y, where "X" indicates number of carbons and "Y" 20 indicates number of double bonds. Fatty acyl groups are major components of many lipids, and their long, non-polar hydrocarbon chain is responsible for the water-insoluble nature of these lipid molecules. The type of covalent linkage of the fatty acyl group to 25 other factors can vary. For example, in biosynthetic reactions they may be covalently bound via a thioester linkage to an acyl carrier protein (ACP) or to CoenzymeA (CoA) , depending on the particular enzymatic reaction. In waxes, fatty acyl groups are linked to fatty alcohols via -, 30 an ester linkage, and triacylglycerols have three fatty acyl groups linked to a glycerol molecule via an ester linkage. The fatty acid composition of an oil determines its physical and chemical properties, and thus its uses. Plants, especially plant species which synthesize large amounts of oils in plant seeds, are an important source of oils both for edible and industrial uses. The fatty acid composition of major oilseeds, ordered here by palmitate content, is shown in Table I. With the exception of laurate (C12:0) sources of coconut endosperm and palm kernel, the common edible oils all basically consist of 16:0, 18:0, 18:1 (oleate), 18:2 (linoleate) , and 18:3 (linolenate) . Table I 12 :0 14: :0 16:0 18:0 18:1 18:2 18: :3 20:1 22:1 rape (HEAR) 3 0.8 9.9 13.5 9. .8 6.8 53.6 rape (LEAR) 4.9 1.4 56.4 24.2 10. .5 sunflower 0. .1 5.8 5.2 16 71.5 0. .2 peanut 6.7 4.3 71.4 11.1 6. .5 safflower 7.6 2 10.8 79.6 coconut 40. .2 15. .5 7.6 2.4 5.2 1.2 oil palm 50, .9 18. .4 8.7 1.9 14.6 1.2 kernel soybean 15.3 3.8 20.7 55.8 9. .4 cotton 1 23.4 2.5 17.9 54.2 oil palm 0, .1 1. .2 46.8 3.8 37.6 mesocarp Plant breeders have successfully modified the yield and fatty acid composition of various plant seed oils through programs of introducing desired traits by plant crosses and selection of progeny carrying the desired trait forward. Application of this technique thus is limited to traits which are found within the same plant species. Alternatively, exposure to mutagenic agents can also introduce traits which may produce changes in the composition of a plant seed oil. However, it is important to note that Fatty Acid Synthesis (FAS) occurs in leaf (chloroplasts) and seed tissue (proplastids) . Thus, although a mutagenesis approach can sometimes result in a desired modification of the composition of a plant seed oil, it is difficult to effect a change which will not alter FAS in other tissues of the plant. A wide range of novel vegetable oils compositions and/or improved means to obtain or manipulate fatty acid compositions, from biosynthetic or natural plant sources, are needed for a variety of intended uses. Plant breeding, even with mutagenesis, cannot meet this need and provide for the introduction of any oil traits which are outside of the target plant's gene pool. Higher plants appear to synthesize fatty acids via a common metabolic pathway in plant plastid organelles (i.e., chloroplasts, proplastids, or other related organelles) as part of the FAS complex. (By fatty acid is meant free fatty acids and acyl-fatty acid groups.) Outside of plastid organelles, fatty acids are incorporated into triacylglycerols (triglycerides) and used in plant membranes and in neutral lipids. In developing seeds, where oils are produced and stored as sources of energy for future use, FAS occurs in proplastids. The production of fatty acids begins in the plastid with the reaction between acetyl-CoA and malonyl-ACP to produce butyryl-ACP catalyzed by the enzyme, β-ketoacyl-ACP synthase III. Elongation of acetyl-ACP to 16- and 18- carbon fatty acids involves the cyclical action of the following sequence of reactions: condensation with a two- carbon unit from malonyl-ACP to form a β-ketoacyl-ACP (β- ketoacyl-ACP synthase) , reduction of the keto-function to an alcohol (β-ketoacyl-ACP reductase) , dehydration to form an enoyl-ACP (β-hydroxyacyl-ACP dehydrase) , and finally reduction of the enoyl-ACP to form the elongated saturated acyl-ACP (enoyl-ACP reductase) . β-ketoacyl-ACP synthase I, catalyzes elongation up to palmitoyl-ACP (C16:0), whereas β-ketoacyl-ACP synthase II catalyz'es the final elongation to stearoyl-ACP (C18:0). The longest chain fatty acids produced by the FAS are typically 18 carbons long. Monounsaturated fatty acids are also produced in the plastid through the action of a desaturase enzyme. A further fatty acid biochemical step occuring in the plastid is the desaturation of stearoyl-ACP (C18:0) to form oleoyl-ACP (C18:l) in a reaction often catalyzed by a Δ-9 desaturase, also often referred to as a "stearoyl-ACP desaturase" because of its high activity toward stearate the 18 carbon acyl-ACP. The desaturase enzyme functions to add a double bond at the ninth carbon in accordance with the following reaction (I) : Stearoyl-ACP + ferredoxin(II) + 0 2 + 2H + -> oleoyl-ACP + ferredoxin (III) + 2H 2 O. Carbon-chain elongation in the plastids can be terminated by transfer of the acyl group to glycerol 3- phosphate, with the resulting glycerolipid retained in the plastidial, "prokaryotic", lipid biosynthesis pathway. Alternatively, specific thioesterases can intercept the prokaryotic pathway by hydrolyzing the newly produced acyl- ACPs into free fatty acids and ACP. Subsequently, the free fatty acids exit the plastids and are incorporated into the "eukaryotic" lipid biosynthesis pathway in the endoplasmic reticulum which is responsible for the formation of phospholipids, triglycerides and other neutral lipids. Previously, plant acyl-ACP thioesterases specific for 18:1-ACP thioesterases were known, and were termed "long- chain"- or "oleoyl"-ACP thioesterases. Encoded by nuclear genes, 18:1-ACP thioesterases are synthesized as preproteins in the cytoplasm and subsequently imported into the plastids. Sequences have been obtained from several angiosperm families and, aside from high variability in the transit peptides, they are very similar to each other. Recently, Pollard, et al . , (Arch , of Biochem. and Biophys . (1991) 254:1-7) identified a medium-chain acyl-ACP thioesterase activity in developing oilseeds of California bay, Umbellularia calif ornica. This activity appears only when the developing cotyledons become committed to the near-exclusive production of triglycerides with lauroyl (12:0) and caproyl (10:0) fatty acids. This work presented the first evidence for a mechanism for medium-chain fatty acid synthesis in plants. The bay thioesterase was subsequently purified by Davies et al . , (Arch. Biochem. Biophys. (1991) 2-90:37-45) which allowed the cloning of a corresponding cDNA which has been used to obtain related clones and to modify the triglyceride composition of plants (Voelker et al . (1992) Science 257: 12- 74; WO 91/16421 and WO 92/20236) . Following transport of free fatty acids to the endoplasmic reticulum, subsequent sequential steps for triglyceride production can occur. For example, polyunsaturated fatty acyl groups such as linoleoyl and α- linolenoyl, are produced as the result of sequential desaturation of oleoyl acyl groups by the action of membrane-bound enzymes. Difficulties in the solubilization of such membrane-bound enzymes has hindered efforts to characterize these enzymes. Additional double bonds are added at the twelve position carbon and thereafter, if added, at the 15 position carbon through the action of Δ-12 desaturase and Δ-15 desaturase, respectively. These "desaturases" thus create mono- or polyunsaturated fatty acids respectively. Triglycerides are then formed by action of the 1-, 2-, and 3- acyl-ACP transferase enzymes glycerol-3-phosphate acyltransferase, lysophosphatidic acid acyltransferase and diacylglycerol acyltransferase. The fatty acid composition of a plant cell is a reflection of the free fatty acid pool and the fatty acids (fatty acyl groups) incorporated into triglycerides as a result of the acyltransferase activities. Thus, in a triglyceride molecule, represented as CH 2 - x CH 2 - Y Formula (I) CH 2 - Z where X, Y, and Z each represents a fatty acyl group which may be the same or different from one another. Various combinations of fatty acyl groups in the different positions in the triglyceride will alter the properties of triglyceride. For example, if the fatty acyl groups are mostly saturated fatty acids, then the triglyceride will be solid at room temperature. In general, however, vegetable oils tend to be mixtures of different triglycerides. The triglyceride oil properties are therefore a result of the combination of triglycerides which make up the oil, which are in turn influenced by their respective fatty acyl compositions. For example, cocoa-butter has certain desirable qualities (mouth feel, sharp melting point, etc.) which are a function of its triglyceride composition. Cocoa-butter contains approximately 24.4% palmitate (16:0), 34.5% stearate (18:0), 39.1% oleate (18:1) and 2% linoleate (18:2) . Thus, in cocoa butter, palmitoyl-oleoyl-stearoyl (POS) (i.e., X, Y and Z, respectively, in Formula I) comprises almost 50% ' of triglyceride composition, with stearate-oleate-stearate (SOS) and palmitoyl-oleoyl- palmitoyl (POP) comprising the major portion of the balance at 39% and 16%, respectively, of the triglyceride composition. Other novel oils compositions of interest might include trierucin (three erucic) or a triglyceride with medium chain fatty acids in each position of the triglyceride molecule. Of particular interest are triglyceride molecules in which palmitoyl is esterified at the X and Z positions of Formula I and a C18 fatty acyl group is at Y. Vegetable oils rich in such PXP (palmitoy1-C18-palmitoyl) molecules have certain desirable qualities in shortening applications, and will find use as additives to solid shortenings to enhance functional effects for retail consumers or industrial food formulators. For example, dipalmitate may be added to shortenings for baking and frying applications to provide proper crystallization of total fat, for example for cake baking applications. In addition, dipalmitate has less effect on melting properties than other possible fat additives, such as distearate, and can be used to produce shortenings having greater "spreadability" within the baking mass. This property of dipalmitate-containing shortenings is of particular interest where the shortening is used as a layering ingredient, for example in pie crusts or sweet goods. In addition, vegetable oils rich in saturated fatty acid content, such as from palmitate and or stearate fatty acids, tend to be solid at room temperature. Such vegetable fats can be used directly in shortenings, margarine and other food "spread" products, without the need for chemical hydrogenation. Hydrogenation is a process to convert unsaturated fatty acids in liquid oils to a saturated form which in turn converts the oil into a solid fat useful in margarine and shortening applications. The cost and any other factors associated with chemical hydrogenation can be avoided if the vegetable oil is engineered to be palmitate and/or stearate rich in the plant seed. Of particular interest in this regard are vegetable oils in which palmitate and stearate are excluded from the sn-2 position of the triglyceride. Furthermore, fatty acids derivatized from vegetable oils containing significant proportions (i.e. 40%) of palmitoyl and stearoyl fatty acyl groups find uses in food emulsification applications due to the desirable softening properties of a palmitate rich fraction that may be obtained from such vegetable oils. Thus, a variety of plant oils modifications are desired, including alternative crop sources for certain oils products and/or means to provide novel fatty acid compositions for plant seed. DESCRIPTION OF THE FIGURES Figure 1. The nucleic acid sequence and translated amino acid sequence of a mango Class I acyl-ACP thioesterase cDNA clone are provided. Figure 2. The nucleic acid sequence and translated amino acid sequence of a mango Class II acyl-ACP thioesterase cDNA clone (MANI-2) are provided. Figure 3. DNA sequence of a partial Class II acyl-ACP thioesterase cDNA clone, 8-2, from leek are provided. Figure 4. DNA sequence of a full length Class II acyl-ACP thioesterase cDNA clone, 9-1, from leek are provided. Figure 5. The nucleic acid sequence and translated amino acid sequence of a bay Class II C12 preferring acyl- ACP thioesterase cDNA clone are provided. Figure 6. The nucleic acid sequence and translated amino acid sequence of a full length Class II Cuphea hookeriana thioesterase (CUPH-2) cDNA clone, CMT7, is shown. Figure 7. The nucleic acid sequence and translated amino acid sequence of a full length Class II Cuphea hookeriana thioesterase (CUPH-1) cDNA clone, CMT9, is shown. Figure 8. The nucleic acid sequence and translated amino acid sequence of an elm C10:0-ACP thioesterase (Class II) partial cDNA clone are provided. Figure 9. Nucleic acid and translated amino acid sequences of a camphor PCR-generated thioesterase encoding sequence is presented. Figure 10. Nucleic acid and translated amino acid sequence of a bay thioesterase clone, Bay D, which represents a second class of bay thioesterase genes, is presented. Figure 11. Nucleic acid sequence and translated amino acid sequence of a Brassica campestris long-chain acyl ACP thioesterase clone is shown. Figure 12. Nucleic acid sequence and translated amino acid sequence of a second mango Class II clone (M4-23) are provided. Figure 13. Fatty acid analysis of Brassica napus 212/86 pooled seed samples from plants transformed with C16:0-ACP thioesterase CUPH-1 {Ch FatBl) construct pCGN4800. Figure 14. Fatty acid analysis of Brassica napus 212/86 pooled seed samples from plants transformed with elm Class II acyl-ACP thioesterase construct pCGN4803. SUMMARY OF THE INVENTION This invention relates to plant thioesterases, specifically plant acyl-ACP thioesterases capable of producing C16:0 (palmitate) fatty acids in transgenic plant seeds. Such acyl-ACP thioesterases are referred to herein as palmitoyl-ACP thioesterases, but may also demonstrate activity on other acyl-ACPs of various chain lengths. By this invention, DNA constructs useful for the expression of a plant palmitoyl-ACP thioesterase in a plant seed cell are described. Such constructs will contain a DNA sequence encoding a plant thioesterase having palmitoyl-ACP hydrolysis activity under the control of regulatory elements capable of preferentially directing expression in seed tissue, as compared with other plant tissues, when such a construct is expressed in a transgenic plant. At least one element of the DNA construct will be heterologous to at least one other of the elements, or when found in a plant cell, at least one element will be heterologous to the plant cell. In a different embodiment, host plant cells containing a first DNA construct capable of expressing a plant palmitoyl-ACP thioesterase, and a second DNA construct capable of expressing an anti-sense stearoyl-acyl ACP desaturase sequence are desired. Such a first DNA construct will contain a DNA sequence encoding the plant palmitoyl-ACP thioesterase of interest under the control of regulatory elements capable of preferentially directing the expression of the plant thioesterase in seed tissue as compared with other plant tissues when such a construct is expressed in a transgenic plant. The second DNA construct will contain a DNA sequence encoding a plant stearoyl-acyl ACP desaturase element positioned in an anti-sense orientation under the control of regulatory elements capable of directing the transcription of the plant stearoyl-acyl ACP desaturase antisense sequence in the plant host seed cell. In yet a different aspect, this invention relates to methods of using a DNA sequence encoding a plant palmitoyl- ACP thioesterase for the modification of the proportion of free fatty acids produced in a plant seed cell. In a like fashion, this invention relates to methods of using such plant palmitoyl-ACP thioesterase encoding sequences to modify the composition of triglycerides, i.e., plant oils, produced by a plant seed to contain increased levels of C16 fatty acyl groups. Plants and plant parts, particularly seeds and oils extracted from such seeds, having such a modified fatty acid composition are contemplated herein. Plant palmitoyl-ACP thioesterase sequences exemplified herein include those obtainable from leek, mango, Cuphea hookeriana and elm. These exemplified thioesterases may be used to obtain other plant palmitoyl-ACP thioesterases of this invention. DETAILED DESCRIPTION OF THE INVENTION By this invention, a mechanism for the increased accumulation of palmitate (C16:0) in plants is demonstrated. Namely, plant acyl-ACP thioesterases having activity toward C16:0-ACP substrates are provided and demonstrated to lead to the production of increased levels of palmitate when expressed in seeds of transgenic plants. Analysis of the encoding sequences and translated amino acid sequences of a number of plant acyl-ACP thioesterases has demonstrated the existence of two evolutionary classes of plant acyl-ACP thioesterases which we have designated as "Class I" or "FatA" (for . fatty acyl transferase type A) and "Class II" (or "FatB") . These classes are not a simple reflection of phylogenetic relationships of the various plants from which the thioesterase encoding sequences were obtained. For example, a Cup-hea hookeriana FatA clone (clone CLT7 in Figure 10 of WO 94/10288) is closely related to safflower FatA clones (sequences provided in Figure 4 of WO 92/20236) . In contrast, a Cuphea hookeriana FatB clone (Figure 7 CUPH-1 clone) is equally distant in evolutionary relationship from the Cuphea hookeriana FatA clone and the safflower FatA clone. Class I thioesterases have been found in mango (Fig.l), safflower, Brassica campestris and Cuphea hookeriana, which sequences are provided in USSN 07/949,102, filed September 21, 1992, now pending, and in WO 92/20236 and WO 94/10288. Class II thioesterases have been discovered in California bay (Figs. 5 & 10), elm (Fig. 8), Cuphea hookeriana (Figs. 6 & 7) , and camphor (Fig. 9). Presently, all known plant medium-chain preferring acyl-ACP thioesterases are of the Class II type, and all known 18:1- ACP thioesterases are of the Class I type. Surprisingly, the Cuphea hookeriana Class II clone represented in Figure 7 was discovered to have preferential activity on palmitoyl-ACP fatty acids and to be capable, when expressed in seeds of transgenic plants, of altering the lipid biosynthesis pathway to increase the content of C16:0 fatty acyl groups incorporated into triglycerides. This discovery was unexpected as this Cuphea species accumulates predominantly C8 and CIO fatty acyl groups in its seed storage lipids. The Cuphea C16:0-ACP thioesterase was demonstrated to be expressed in various plant tissues, including leaves, stems and roots, whereas a different Cuphea Class II thioesterase (CUPH-2; Figure 6), which was preferentially active on C8:0 and C10:0 substrates, was expressed preferentially in seed tissues. A Class II acyl-ACP thioesterase clone was also discovered in an Arabidopsis silique cDNA library based on homology of a randomly cloned cDNA fragment to the bay C12:0-ACP thioesterase clone. A full length cDNA clone encoding the Arabidopsis thioesterase was obtained and demonstrated to encode a thioesterase having activity on a range of acyl-ACP substrates, including 16:0-ACP. Thus, it is now seen that Class II type fatty acyl-ACP thioesterase sequences may be found in plant species not known to accumulate medium chain length fatty acids. Such acyl-ACP thioesterase sequences may be examined, for example by expression in E. coli or plant cells, to identify those clones having significant or preferential activity on C16:0-ACP substrates. In this application, cDNA sequence of the Cuphea hookeriana palmitoyl-ACP thioesterase (Fig. 7) is provided, as well as additional C16:0-ACP thioesterase sequences from mango (Figs. 2 and 12), and leek (Figs. 3 & 4) . In addition, DNA sequence of an elm acyl-ACP thioesterase clone is provided (Fig. 8) . Expression of the elm clone in transgenic plant seeds results in an increased proportion of C16:0 fatty acyl groups in the triglycerides, as well as a smaller increase in the proportion of C14:0 fatty acyl groups. All of these acyl-ACP thioesterase sequences, which may be used to increase palmitoyl content of transgenic plant seed oil, show significant sequence identity with other Class II plant thioesterase proteins. Plants having significant presence of C16:0 fatty acids therein are also candidates to obtain naturally- derived palmitoyl-ACP preferring plant thioesterases. However, it is also recognized that other plant sources which do not have a significant presence of palmitate may also be screened as other enzyme sources. As discussed above, it is now believed that proteins demonstrating palmitoyl-ACP thioesterase activity will show a high degree of homology at the DNA and amino acid level, with a particular class of acyl-ACP thioesterases known as "Class II" thioesterases herein. Thus, it is possible to readily screen for additional plant palmitoyl-ACP thioesterases in any plant of interest. As described in more detail in the following Examples, acyl-ACP thioesterases having activity on C16:0 fatty acids may be expressed in the seeds of target transgenic plants with a resultant increase in the percentage of C16:0 fatty acids in the seed triglycerides. For example, expression of the Cuphea hookeriana clone, CUPH-1 (also known as Ch FatBl) , in the seeds of transgenic Brassica napus plants results in the transformants having C16:0 seed fatty acid contents ranging from 7-26 mol% in pooled seed samples. Analysis of individual seeds from selected transformants revealed C16:0 fatty acid contents of up to 34 mol%. Similarly, expression of a mango Class II clone (MANI-2 sequence shown in Figure 2) results in transformants having C16:0 seed fatty acid contents ranging from 7-16 mol% in pooled seed samples. Expression of a Class II acyl-ACP thioesterase from elm (Figure 8) yields transgenic Brassica napus plants having C16:0 pooled seed fatty acid contents ranging from 15-32 mol%. In addition, C14:0 fatty acid contents are increased in these seeds to levels ranging from 2-12 mol%, and increases in C10:0 (up to 4 mol%) and C12.-0 (up to 1.5 mol%) are also observed. Background levels of C12:0, C14:0, and C16:0 fatty acids in non- transgenic seeds of the transformed variety are approximately 0.02, 0.14 and 6 mol%, respectively. C10:0 fatty acids in non-transgenic seeds are typically determined as being present at less than 1%. (The C10.-0 background value in controls may be the result of machine reading of a non-C10:0 peak, as the levels vary from 0 to 1% in analysis of non-transgenic Brassica napus plant seed. ) TAG positional analysis of the described transgenic plants is expected to reveal the exclusion of the C16:0 fatty acyl groups from the sn-2 position of the seed triglycerides. The seed microsomal 2-acyltransferase in the Brassica plants does not demonstrate efficient activity on substrates other than C18:l-CoA. Furthermore, seed lipids in plants transformed with a bay C12:0-ACP thioesterase contain up to 50 mol% C12:0 fatty acids, but the C12:0 acyl groups were located almost exclusively in the sn-1 and sn-3 positions of the triglycerides (WO 92/20236). A plant palmitoyl-ACP thioesterase DNA sequence of this invention encodes for amino acids, in the form of a protein, polypeptide or peptide fragment, which amino acids demonstrate the ability to catalyze the production of C16:0 free fatty acid (i.e., palmitate) from palmitoyl-ACP substrates under plant enzyme reactive conditions. By "enzyme reactive conditions" is meant that any necessary conditions are available in an environment (i.e., such factors as temperature, pH, lack of inhibiting substances) which will permit the enzyme to function. One skilled in the art will readily recognize that antibody preparations, nucleic acid probes (DNA and RNA) and the like may be prepared and used to screen and recover "homologous" or "related" palmitoyl-ACP thioesterases from a variety of plant sources. Typically, nucleic acid probes are labeled to allow detection, preferably with radioactivity although enzymes or other methods may also be used. For immunological screening methods, antibody preparations either monoclonal or polyclonal are utilized. Polyclonal antibodies, although less specific, typically are more useful in gene isolation. For detection, the antibody is labeled using radioactivity or any one of a variety of second antibody/enzyme conjugate systems that are commercially available. Examples of some of the available antibody detection systems are described by Oberfilder (Focus (1989) BRL Life Technol. , Inc. ,12:1-5) . Homologous sequences are found when there is an identity of sequence and may be determined upon comparison of sequence information, nucleic acid or amino acid, or through hybridization reactions between a known thioesterase and a candidate source. Conservative changes, such as Glu/Asp, Val/Ile, Ser/Thr, Arg/Lys and Gln/Asn may also be considered in determining sequence homology. Typically, a lengthy nucleic acid sequence may show as little as 50-60% sequence identity, and more preferably at least about 70% sequence identity, between the target sequence and the given plant thioesterase of interest excluding any deletions which may be present, and still be considered related. Amino acid sequences are considered homologous by as little as 25% sequence identity between the two complete mature proteins. ( See generally, Doolittle, R.F., OF URFS and ORFS (University Science Books, CA, 1986.) Although all plant thioesterases show approximately 50% sequence identity at the nucleic acid level, between members of Class II the percentage of sequence identity jumps to at least 60%. At the amino acid level, the region corresponding from approximately amino acid 60 through amino acid 150 in the California bay will show a very high degree of conservation between members of Class II thioesterases. In order to obtain additional palmitoyl-ACP thioesterases, a genomic or other appropriate library prepared from the candidate plant source of interest is probed with conserved sequences from one or more Class II plant thioesterase(s) to identify homologously related sequences. Positive clones are then analyzed by restriction enzyme digestion and/or sequencing. When a genomic library is used, one or more sequences may be identified providing both the coding region, as well as the transcriptional regulatory elements of the thioesterase gene from such plant source. Probes can also be considerably shorter than the entire sequence. Oligonucletodies may be used, for example, but should be at least about 10, preferably at least about 15, more preferably at least 20 nucleotides in length. When shorter length regions are used for comparison, a higher degree of sequence identity is required than for longer sequences. Shorter probes are often particularly useful for polymerase chain reactions (PCR) , especially when highly conserved sequences can be identified. (See, Gould, et al . , PNAS USA (1989) 55:1934-1938.) When longer nucleic acid fragments are employed (>100 bp) as probes, especially when using complete or large cDNA sequences, one can still screen with moderately high stringencies (for example using 50% formamide at 37°C with minimal washing) in order to obtain signal from the target sample with 20-50% deviation, i.e., homologous sequences. (For additional information regarding screening techniques see Beltz, et al . , Meth. Enzy ology (1983) 100:266-285) . Again, not only can sequences such as shown in Figs. 2-10 be used to identify homologous plant palmitoyl-ACP thioesterases, but the resulting sequences obtained therefrom may also provide a further method to obtain plant palmitoyl-ACP thioesterases from other plant sources. In particular, PCR may be a useful technique to obtain related plant thioesterases from sequence data provided herein. One skilled in the art will be able to design oligonucleotide probes based upon sequence comparisons or regions of typically highly conserved sequence. Once the nucleic acid sequence is obtained, the transcription, or transcription and translation (expression) , of the plant palmitoyl-ACP thioesterase in a host cell is desired to produce a ready source of the enzyme and/or modify the composition of fatty acids and/or triglycerides found therein. Other useful applications may be found when the host cell is a plant host cell, in vitro and in vivo. For example, by increasing the amount of palmitoyl-ACP preferring thioesterase available to the plant FAS complex, an increased percentage of palmitate may be provided. In a like manner, for some applications, by decreasing the amount of stearoyl-ACP desaturase available to the plant FAS complex in conjunction with an increase of the amount of palmitoyl-ACP thioesterase available, a substantial increase in the saturated fatty acids stearate and palmitate may be obtained. The nucleic acid sequences which encode plant palmitoyl-ACP thioesterases may be used in various constructs, for example, as probes to obtain further sequences. Alternatively, these sequences may be used in conjunction with appropriate regulatory sequences to increase levels of the respective thioesterase of interest in a host cell for recovery or study of the enzyme in vitro or in vivo or to decrease levels of the respective thioesterase of interest for some applications when the host cell is a plant entity, including plant cells, plant parts (including but not limited to seeds, cuttings or tissues) and plants. A nucleic acid sequence encoding a plant palmitoyl-ACP thioesterase of this invention may include genomic, cDNA or mRNA sequence. By "encoding" is meant that the sequence corresponds to a particular amino acid sequence either in a sense or anti-sense orientation. By "extrachromosomal" is meant that the sequence is outside of the plant genome of which it is naturally associated. By "recombinant" is meant that the sequence contains a genetically engineered modification through manipulation via mutagenesis, restriction enzymes, and the like. A cDNA sequence may or may not contain pre-processing sequences, such as transit peptide sequences. Transit peptide sequences facilitate the delivery of the protein to a given organelle and are cleaved from the amino acid moiety upon entry into the organelle, releasing the "mature" sequence. The use of the precursor plant palmitoyl-ACP thioesterase DNA sequence is preferred in plant cell expression cassettes. Other plastid transit peptide sequences, such as a transit peptide of seed ACP, may be employed to translocate the plant palmitoyl-ACP thioesterase of this invention to various organelles of interest. See, U.S. Serial No. 07/437,764, filed 11/15/89 and European Patent Application Publication No. 189,707. In a like manner, once a given plant palmitoyl-ACP thioesterase transit peptide is obtained, it may be used to translocate sequences other than its native coding region. Sequence comparisons to the various Class II thioesterase clones provides information about the transit peptide region for these thioesterase clones. The mature N-terminus of the purified bay C12:0- ACP thioesterase was originally determined by amino acid sequencing as residue 84 (Voelker et al . (1992) Science 257: 12-14- ) . However, this would place the most N-terminal region conserved between all FatB representatives (residues 60-82 for bay thioesterase, Fig. 5) into the transit peptide. This sequence would be unusual for stromal transit peptides, not only because of the high degree of sequence conservation, but also because it contains a hydrophobic domain resembling thylakoid transit peptides. We now believe that this conserved region is not part of the transit sequence, and that it is included in the N- terminal portion of the processed mature protein. Consistent with this hypothesis, expression of bay thioesterase in transgenic canola seeds produces a bay thioesterase having a M r of 40kD, i.e. larger than the 34kD purified seed protein. The 40kD form may represent the in vivo state of bay thioesterase, with the 34kD from resulting from limited N-terminal proteolysis during purification. Furthermore, as discussed above the complete genomic sequence of the plant palmitoyl-ACP thioesterase may be obtained by the screening of a genomic library with a probe, such as a cDNA probe, and isolating those sequences which regulate expression in seed tissue. In this manner, the transcription and translation initiation regions, introns, and/or transcript termination regions of the plant palmitoyl-ACP thioesterase may be obtained for use in a variety of DNA constructs, with or without the thioesterase structural gene. Thus, nucleic acid sequences corresponding to the plant palmitoyl-ACP thioesterase of this invention may also provide signal sequences useful to direct transport into a plastid, 5' upstream non-coding regulatory regions (promoters) having useful tissue and timing profiles, 3' downstream non-coding regulatory region useful as transcriptional and translational regulatory regions and may lend insight into other features of the gene. Once the desired plant palmitoyl-ACP thioesterase nucleic acid sequence is obtained, it may be manipulated in a variety of ways. Where the sequence involves non-coding flanking regions, the flanking regions may be subjected to resection, mutagenesis, etc. Thus, transitions, transversions, deletions, and insertions may be performed on the naturally occurring sequence. In addition, all or part of the sequence may be synthesized. In the structural gene, one or more codons may be modified to provide for a modified amino acid sequence, or one or more codon mutations may be introduced to provide for a convenient restriction site or other purpose involved with construction or expression. The structural gene may be further modified by employing synthetic adapters, linkers to introduce one or more convenient restriction sites, or the like. The nucleic acid or amino acid sequences encoding a plant palmitoyl-ACP thioesterase of this invention may be combined with other non-native, or "heterologous", sequences in a variety of ways. By "heterologous" sequences is meant any sequence which is not naturally found joined to the plant palmitoyl-ACP thioesterase, including, for example, combinations of nucleic acid sequences from the same plant which are not naturally found joined together. The DNA sequence encoding a plant palmitoyl-ACP thioesterase of this invention may be employed in conjunction with all or part of the gene sequences normally associated with the thioesterase. In its component parts, a DNA thioesterase encoding sequence is combined in a DNA construct having, in the 5' to 3 ' direction of transcription, a transcription initiation control region capable of promoting transcription and translation in a host cell, the DNA sequence encoding plant palmitoyl-ACP thioesterase and a transcription and translation termination region. Potential host cells include both prokaryotic and eukaryotic cells. A host cell may be unicellular or found in a multicellar differentiated or undifferentiated organism depending upon the intended use. Cells of this invention may be distinguished by having a plant palmitoyl- ACP thioesterase foreign to the wild-type cell present therein, for example, by having a recombinant nucleic acid construct encoding a plant palmitoyl-ACP thioesterase therein. Depending upon the host, the regulatory regions will vary, including regions from viral, plasmid or chromosomal genes, or the like. For expression in prokaryotic or eukaryotic microorganisms, particularly unicellular hosts, a wide variety of constitutive or regulatable promoters may be employed. Expression in a microorganism can provide a ready source of the plant enzyme. Among transcriptional initiation regions which have been described are regions from bacterial and yeast hosts, such as E. coli, B. subtilis, Sacchromyces cerevisiae, including genes such as beta-galactosidase, T7 polymerase, tryptophan E and the like. For the most part, the constructs will involve regulatory regions functional in plants which provide for modified production of plant palmitoyl-ACP thioesterase, and possibly, modification of the fatty acid composition. The open reading frame, coding for the plant palmitoyl-ACP thioesterase or functional fragment thereof will be joined at its 5 ' end to a transcription initiation regulatory region such as the wild-type sequence naturally found 5 ' upstream to the thioesterase structural gene. Numerous other transcription initiation regions are available which provide for a wide variety of constitutive or regulatable, e.g., inducible, transcription of the structural gene functions. Among transcriptional initiation regions used for plants are such regions associated with the structural genes such as for nopaline and mannopine synthases, or with napin, ACP promoters and the like. The transcription/translation initiation regions corresponding to such structural genes are found immediately 5' upstream to the respective start codons. In embodiments wherein the expression of the thioesterase protein is desired in a plant host, the use of all or part of the complete plant palmitoyl-ACP thioesterase gene is desired; namely all or part of the 5' upstream non-coding regions (promoter) together with the structural gene sequence and 3 ' downstream non-coding regions may be employed. If a different promoter is desired, such as a promoter native to the plant host of interest or a modified promoter, i.e., having transcription initiation regions derived from one gene source and translation initiation regions derived from a different gene source, including the sequence encoding the plant palmitoyl-ACP thioesterase of interest, or enhanced promoters, such as double 35S CaMV promoters, the sequences may be joined together using standard techniques. For such applications when 5 ' upstream non-coding regions are obtained from other genes regulated during seed maturation, those preferentially expressed in plant embryo tissue, such as ACP and napin-derived transcription initiation control regions, are desired. Such "seed- specific promoters" may be obtained and used in accordance with the teachings of U.S. Serial No. 07/147,781, filed 1/25/88 (now U.S. Serial No. 07/550,804, filed 7/9/90) , and U.S. Serial No. 07/494,722 filed on or about March 16, 1990 having a title "Novel Sequences Preferentially Expressed In Early Seed Development and Methods Related Thereto, " which references are hereby incorporated by. reference. Transcription initiation regions which are preferentially expressed in seed tissue, i.e., which are undetectable in other plant parts, are considered desirable for fatty acid modifications in order to minimize any disruptive or adverse effects of the gene product. Regulatory transcript termination regions may be provided in DNA constructs of this invention as well. Transcript termination regions may be provided by the DNA sequence encoding the plant palmitoyl-ACP thioesterase or a convenient transcription termination region derived from a different gene source, for example, the transcript termination region which is naturally associated with the transcript initiation region. Where the transcript termination region is from a different gene source, it will contain at least about 0.5 kb, preferably about 1-3 kb of sequence 3 ' to the structural gene from which the termination region is derived. Plant expression or transcription constructs having a plant palmitoyl-ACP thioesterase as the DNA sequence of interest for increased or decreased expression thereof may be employed with a wide variety of plant life, particularly, plant life involved in the production of vegetable oils for edible and industrial uses. Most especially preferred are temperate oilseed crops. Plants of interest include, but are not limited to, rapeseed (Canola and High Erucic Acid varieties) , sunflower, safflower, cotton, Cuphea, soybean, peanut, coconut and oil palms, and corn. Depending on the method for introducing the recombinant constructs into the host cell, other DNA sequences may be required. Importantly, this invention is applicable to dicotyledyons and monocotyledons species alike and will be readily applicable to new and/or improved transformation and regulation techniques. The method of transformation is not critical to the instant invention; various methods of plant transformation are currently available. As newer methods are available to transform crops, they may be directly applied hereunder. For example, many plant species naturally susceptible to Agrobacterium infection may be successfully transformed via tripartite or binary vector methods of Agrobacterium mediated transformation. In addition, techniques of microinjection, DNA particle bombardment, electroporation have been developed which allow for the transformation of various monocot and dicot plant species. In developing the DNA construct, the various components of the construct or fragments thereof will normally be inserted into a convenient cloning vector which is capable of replication in a bacterial host, e.g., E. coli . Numerous vectors exist that have been described in the literature. After each cloning, the plasmid may be isolated and subjected to further manipulation, such as restriction, insertion of new fragments, ligation, deletion, insertion, resection, etc., so as to tailor the components of the desired sequence. Once the construct has been completed, it may then be transferred to an appropriate vector for further manipulation in accordance with the manner of transformation of the host cell. Normally, included with the DNA construct will be a structural gene having the necessary regulatory regions for expression in a host and providing for selection of transformant cells. The gene may provide for resistance to a cytotoxic agent, e.g. antibiotic, heavy metal, toxin, etc. , complementation providing prototrophy to an auxotrophic host, viral immunity or the like. Depending upon the number of different host species the expression construct or components thereof are introduced, one or more markers may be employed, where different conditions for selection are used for the different hosts. It is noted that the degeneracy of the DNA code provides that some codon substitutions are permissible of DNA sequences without any corresponding modification of the amino acid sequence. As mentioned above, the manner in which the DNA construct is introduced into the plant host is not critical to this invention. Any method which provides for efficient transformation may be employed. Various methods for plant cell transformation include the use of Ti- or Ri-plasmids, microin ection, electroporation, DNA particle bombardment, liposome fusion, DNA bombardment or the like. In many instances, it will be desirable to have the construct bordered on one or both sides by T-DNA, particularly having the left and right borders, more particularly the right border. This is particularly useful when the construct uses A. tumefaciens or A. rhizogenes as a mode for transformation, although the T-DNA borders may find use with other modes of transformation. Where Agrobacterium is used for plant cell transformation, a vector may be used which may be introduced into the Agrobacterium host for homologous recombination with T-DNA or the Ti- or Ri-plasmid present in the Agrobacterium host. The Ti- or Ri-plasmid containing the T-DNA for recombination may be armed (capable of causing gall formation) or disarmed (incapable of causing gall formation) , the latter being permissible, so long as the vir genes are present in the transformed Agrobacterium host. The armed plasmid can give a mixture of normal plant cells and gall. In some instances where Agrobacterium is used as the vehicle for transforming plant cells, the expression construct bordered by the T-DNA border(s) will be inserted into a broad host spectrum vector, there being broad host spectrum vectors described in the literature. Commonly used is pRK2 or derivatives thereof. See, for example, Ditta et al . , PNAS USA, (1980) 77:7347-7351 and EPA 0 120 515, which are incorporated herein by reference. Included with the expression construct and the T-DNA will be one or more markers, which allow for selection of transformed Agrobacterium and transformed plant cells. A number of markers have been developed for use with plant cells, such as resistance to chloramphenicol, the aminoglycoside G418, hygromycin, or the like. The particular marker employed is not essential to this invention, one or another marker being preferred depending on the particular host and the manner of construction. For transformation of plant cells using Agrobacterium, explants may be combined and incubated with the transformed Agrobacterium for sufficient time for transformation, the bacteria killed, and the plant cells cultured in an appropriate selective medium. Once callus forms, shoot formation can be encouraged by employing the appropriate plant hormones in accordance with known methods and the shoots transferred to rooting medium for regeneration of plants. The plants may then be grown to seed and the seed used to establish repetitive generations and for isolation of vegetable oils. Once a transgenic plant is obtained which is capable of producing seed having a modified fatty acid composition, traditional plant breeding techniques, including methods of mutagensis, may be employed to further manipulate the fatty acid composition. Alternatively, additional foreign fatty acid modifying DNA sequence may be introduced via genetic engineering to further manipulate the fatty acid composition. It is noted that the method of transformation is not critical to this invention. However, the use of genetic engineering plant transformation methods, i.e., the power to insert a single desired DNA sequence, is critical. Heretofore, the ability to modify the fatty acid composition of plant oils was limited to the introduction of traits that could be sexually transferred during plant crosses or viable traits generated through mutagensis. Through the use of genetic engineering techniques which permits the introduction of inter-species genetic information and the means to regulate the tissue-specific expression of endogenous genes, a new method is available for the production of plant seed oils with modified fatty acid compositions. In addition, there is the potential for the development of novel plant seed oils upon application of the tools described herein. One may choose to provide for the transcription or transcription and translation of one or more other sequences of interest in concert with the expression of a plant palmitoyl-ACP thioesterase in a plant host cell. In particular, the reduced expression of stearoyl-ACP desaturase in combination with expression of a plant palmitoyl-ACP thioesterase may be perferred in some applications. When one wishes to provide a plant transformed for the combined effect of more than one nucleic acid sequence of interest, typically a separate nucleic acid construct will be provided for each. The constructs, as described above contain transcriptional or transcriptional or transcriptional and translational regulatory control regions. One skilled in the art will be able to determine regulatory sequences to provide for a desired timing and tissue specificity appropriate to the final product in accord with the above principles set forth as to the respective expression or anti-sense constructs. When two or more constructs are to be employed, whether they are both related to the same fatty acid modifying sequence or a different fatty acid modifying sequence, it may be desired that different regulatory sequences be employed in each cassette to reduce spontaneous homologous recombination between sequences. The constructs may be introduced into the host cells by the same or different methods, including the introduction of such a trait by crossing transgenic plants via traditional plant breeding methods, so long as the resulting product is a plant having both characteristics integrated into its genome. A plant stearoyl-ACP desaturase of this invention includes any sequence of amino acids, such as a protein, polypeptide, or peptide fragment, obtainable from a plant source which is capable of catalyzing the insertion of a first double bond into a fatty acyl-ACP moiety in a plant host cell, i.e., in vivo, or in a plant cell-like environment, i.e. in vitro. "A plant cell-like environment" means that any necessary conditions are available in an environment (i.e., such factors as temperatures, pH, lack of inhibiting substances) which will permit the enzyme to function In particular, this invention relates to enzymes which add such a first double bond at the ninth carbon position in a fatty acyl-ACP chain. There may be similar plant desaturase enzymes of this invention with different specificities, such as the Δ- 12 desaturase of carrot. By decreasing the amount of desaturase, an increased percentage of saturated fatty acids is provided. Using anti-sense, transwitch, ribozyme or some other stearoyl-ACP desaturase reducing technology, a decrease in the amount of desaturase available to the plant cell is produced, resulting in a higher percentage of saturates such as one or more of laurate (C12:0), myristate (C14:0), palmitate (C16:0), stearate (C18:0), arachidate (C20:0), behemate (C22:0) and lignocerate (C24:0) . In rapeseed reduced desaturase results in increased stearate levels and total saturates. Of special interest is the production of triglycerides having increased levels of palmitate or palmitate and stearate. In addition, the production of a variety of ranges of such saturates is desired. Thus, plant cells having lower and higher levels of palmitate or palmitate and stearate fatty acids are contemplated. For example, fatty acid compositions, including oils, having a 10% level of palmitate and stearate as well as compositions designed to have up to an appropriate 60% level of palmitate and stearate or other such modified fatty acid(s) composition are contemplated. Oils with increased percentages of palmitate or palmitate and stearate, are desired. Increased stearate percentages (by weight) ranging from native up to 25 fold are described. By manipulation of various aspects of the DNA constructs (e.g., choice of promoters, number of copies, etc.) and traditional breeding methods, one skilled in the art may achieve even greater levels of stearate. By combination of the plant desaturase sequence in combination with the expression of a plant palmitoyl-ACP thioesterase in seed tissue, an increased percentage of palmitate and stearate can be achieved in rapeseed and other plant species. DNA sequence of C. tinctorius desaturase gene, as well as DNA sequences of desaturase gene from a Ricinus, a Brassica and a Simmondsia plant are found in USSN 07/949,102, filed September 21, 1992, now pending. The invention now being generally described, it will be more readily understood by reference to the following examples which are included for purposes of illustration only and are not intended to limit the present invention. EXAMPLES Example 1 Thioesterase Gene Sequences A. Mango A mango cDNA bank is prepared using the methods as described in Stratagene Zap cDNA synthesis kit (Stratagene; La Jolla, CA) . Mango embryos are collected from green (mature but not soft) mangos purchased from a grocery store. Total RNA is isolated from the embryos by modifying the DNA isolation method of Webb and Knapp (Plant Mol . Biol . Reporter (1990) 5:180-195) . Buffers include: REC: 50mM TrisCl pH 9, 0.7 M NaCl, 10 mM EDTA pH8, 0.5% CTAB. REC+: Add B-mercaptoethanol to 1% immediately prior to use. RECP: 50 mM TrisCl pH9, 10 mM EDTA pH8, and 0.5% CTAB. RECP+: Add B-mercaptoethanol to 1% immediately prior to use. For extraction of 1 g of tissue, 10ml of REC+ and 0.5 g of PVPP is added to tissue that has been ground in liquid nitrogen and homogenized. The homogenized material is centrifuged for 10 min at 1200 rpm. The supernatant is poured through miracloth onto 3ml cold chloroform and homogenized again. After centrifugation, 12,000 RPM for 10 min, the upper phase is taken and its volume determined. An equal volume of RECP+ is added and the mixture is allowed to stand for 20 min. at room temperature. The material is centrifuged for 20 min. at 10,000 rpm twice and the supernatant is discarded after each spin. The pellet is dissolved in 0.4 ml of 1 M NaCl (DEPC) and extracted with an equal volume of phenol/chloroform. Following ethanol preciptation, the pellet is dissolved in 1 ml of DEPC water. Briefly, the cloning method for cDNA synthesis is as follows. First strand cDNA synthesis is according to Statagene Instruction Manual with some modifications according to Robinson, et al . (Methods in Molecular and Cellular Biology (1992) 3:118-127) . In particular, 30μg of LiCl precipitated total RNA is used instead of 5μg of poly(A)+ RNA and the reaction is incubated at 45°C rather than 37°C for 1 hour. The library is screened by plating ~15000pfu on LE392 E. coli cells per 150mm NZY plate in 0.9% NZY top agarose to provide appoximately 300,000 plaques for screening. Plaque lifts onto Colony/Plaque Screen (NEN) and denaturization, neutralization and baking was done as described (Stratagene) . Filters are prehybridized at room temperature in 50% formamide, 5X SSC, 10X Denharts, 0.1% (w/v) SDS, 5mM Na 2 EDTA, 0.lmg/ml denatured salmon sperm DNA (from 2 hours to overnight) in plastic boxes with teflon screens between the filters. Hybridization is conducted at room temperature in the same buffer as above with added 10%(w/v) dextran sulfate and probe. Probe used is a fragment of the Brassica Class I thioesterase containing nucleotides 27- 1191 of the sequence shown in Figure 11. Filters are washed for 2 minutes with IX SSC/0.1% SDS, and twice for 20 minutes each wash with 0.IX SSC/0.1% SDS. Filters are exposed to X-ray film overnight. Twenty-six hybridizing plaques were identified. Plaque purification and phagemid excision were conducted as described in Stratagene Zap cDNA Synthesis Kit instructions. Of 25 purified phagemids analyzed, thioesterase clones from two classes were identified by DNA sequence analysis. Nine of the clones encode Class I thioesterase and sixteen encode Class II thioesterase. The sequence of the longest member of each class are shown in Figures 1 (Class I) and 2 (Class II) . The mango library was rescreened for additional thioesterase clones. Phage (147200 pfu) are plated on 16 NZY plates (9200 pfu/plate) as described above. For each plate, approximately 9200 phage are mixed with 600μl of LE392 cells (1 O.D., in 10 mM MgS04) and incubated at 37°C for 20 min. Cells are then plated by adding 6 mis of 0.9% NZY top agarose (55°C) and poured directly onto an NZY plate that has been warmed to 37°C. Plates are incubated at 37°C overnight. Plaque lifts onto Colony/Plaque Screen (NEN) and denaturization and neutralization are as described above (Stratagene) . Filters are air dried overnight, prehybed and hybridized as described above. A PCR fragment of the mango class II thioesterase (Figure 2) is used as the probe. The probe is prepared from pCGN5214 plasmid as the PCR template using the following primers: 4316 5' CGTCTAGACATATGCCAAGGACTTTTATTAAC 3' 4343 5' CGGAATTCCGAGACTGCAGTAAGGCTAATC 3'. The following PCR conditions are used: 94°C 15 sec, 50°C 30 sec, 72°C 30 sec. for 30 cycles in a Perkin-Elmer GeneAmp PCR System 9600 thermocycler. The approximately 1200 base pair resulting PCR fragment is radiolabeled using the Stratagene "Prime-it II Random Prime Labeling Kit". The labeled DNA fragments are passed through a Sephadex-G50 spin column to separate unincorporated dNTPs. The labeled probe is added to the hybridization solution. After overnight incubation in this solution at room temperature, the membranes are washed once for 15 minutes at room temperature with IX SSC plus 0.1% SDS followed by two washes with 0.1X SSC plus 0.1% SDS under the same conditions. The membranes are exposed to X-ray film for 3 days. A total of 28 positive signals were seen on the X- ray films, and the corresponding clones were picked for further sceening. By DNA sequence analysis, 17 of the clones were identified as encoding acyl-ACP thioesterases. All but one of the clones were identical to the mango Class II clone provided in Figure 2. Sequence of the other clone, M4-23, which is closely related to the previous mango Class II thioesterase clone, is provided in Figure 12. B. Leek Similarly to the methods described above for isolation of mango thioesterase sequences, a leek cDNA library in lambda ZAP (Stratagene) is screened for thioesterase clones. Phage (2 x 10 5 pfu) are plated on 10 NZY plates (20,000 pfu/plate) as described above. For each plate, approximately 20,000 phage are mixed with 600 μl of LE392 cells (1 O.D., in 10 mM MgS0 4 ) and incubated at 37°C for 15 min. Cells are then plated by adding 7 ml of 0.9% NZY top agarose (55°C) and poured directly onto an NZY plate (see above) . Plates are incubated at 37°C overnight. Following incubation at 4°C for 2 hours, phage are lifted onto colony /plaque screen membranes (New England Nuclear) as described above. Double lifts are made to prevent false positve signals. After lifting, the membranes are denatured, neutralized, and rinsed as described above. The phage DNA is then crosslinked to the membrane using an UV Stratalinker 2400 (Stratagene) . Membranes are then submerged in prehybridization solution containing 50% formamide and 5X SSC, and incubated at room temperature for 2 hours (with shaking) . Three thioesterase DNA fragments were chosen for use in hybridization reactions. The mixed probe contained (1) a bay 12:0 thioesterase fragment (gel-purified 400 bp PstI fragment of the bay cDNA shown in Figure 5); (2) mango Class II thioesterase (Figure 2) and (3) Brassica Class I thioesterase (Figure 11) . Approximately 100 ng of each purified DNA fragment is radio-labelled using the Pharmacia "Ready-To-Go" DNA labeling kit. Following the manufacturer's instructions, 50 μCi 32 P-dCTP is used for each reaction. The labeled DNA fragments are passed through a Sephadex-G25 spin column to separate unincorporated dNTPs. The three labeled probes are mixed and added to 75 ml of hybridization solution (prehybridization solution containing 10% dextran sulfate) . After overnight incubation in this solution at room temperature, the membranes are washed twice for 15 minutes at room temperature with IX SSC and 0.1% SDS, followed by two washes with 0.1X SSC plus 0.1% SDS under the same conditions. The membranes are exposed to X-ray film overnight. A total of 28 positive signals were seen on the X-ray films. Five of the clones (designated as LTE 6-1, 8-2, 8-3, 9-1, 9-4) were picked for further screening. Phage are eluted from the plates in the area of the positive signals by soaking a core of the agarose in 200 μl of SM buffer. Each phage was plated out as described above at a titer of about 100 pfu/plate. Phage DNA were transferred to membranes, denatured, neutralized, and cross-linked as described above. The membranes were hybridized with the same probes used in the initial screening, and washed under identical conditions. The majority of the plaques were also positive in the secondary screening. Well-isolated, positive plaques were picked and subjected to excision using the Exassist/SOLR system from Stratagene. The cloned DNA inserts, now in a pBluescript double-stranded phagemid, were purified, and sequenced using an ABS automated sequencer. DNA sequences of the five selected clones revealed that clone 8-2, 8-3 and 9-4 are identical, while 9-1 and 6- 1 appear to be identical and are very closely related, if not identical to the other three clones. The leek thioesterases encoded by these cDNA clones show high homology to Class II thioesterases. Preliminary nucleic acid sequence and translated amino acid sequence of 8-2 and 9-1 are provided in Figures 3 and 4. C. Cuphea Acyl-ACP thioesterase clones are isolated from Cuphea hookeriana as described in WO 94/10288. Sequence of Class II C. hookeriana clones are provided in Figures 6 and 7. D. Elm A Class II acyl-ACP thioesterase clone is isolated from elm as described in WO 94/10288. DNA sequence of this clone is provided in Figure 8. Example 2 Expression of Acyl-ACP Thioesterases in E. coli To determine substrate preference of thioesterases encoded by the isolated sequences, recombinant DNA constructs for expression in E. coli are prepared. A. Mango To insert the mango Class II thioesterase encoding sequence into a pET3a expression vector, the mango Class II thioesterase cDNA, MANI-2 is digested with EcoKL . The linearized plasmid is used as a PCR template using the following primers: 4317 = 5' CGTCTAGACATATGCTTGACTGGAAACC 3 ' ; 4343 = 5' CGGAATTCCGAGACTGCAGTAAGGCTAATC 3'. The following PCR conditions are used: 94°C 1 min. , 60°C 30 sec, 72°C 2 min for 30 cycles in a Perkin-Elmer GeneAmp PCR System 9600 thermocycler. The approximately 1100 base pair resulting PCR fragment contains an Ndel site immediately upstream of the postulated mature protein start codon (Leucine 112) . The fragment is cloned into a TA1000 vector (Invitrogen) to generate pCGN5217. pCGN5217 is digested with Ndel and EcόRl to produce a fragment containing the majority of the mature mango thioesterase coding sequence. The Ndel/EcόRI fragment is inserted into Ndel/.EcoRI digested plasmid expression vector pET3a (Νovagen; Madison, WI) resulting in pCGΝ5218. For expression analysis, pCGN5218 is used to transform E. coli strain BL2KDE3) which contains a T7 polymerase (Novagen) . An additional construct is prepared which includes a different mature protein N-terminus candidate (leucine 88) . The PCR is conducted using primers 4466 and 4464 with the following PCR conditions: 94°C 1 min., 60°C 30 sec, 72°C 2 min for 30 cycles. The PCR fragment is cloned into CloneAmp System (GIBCO BRL) , and the resulting plasmid is digested with Ndel and .EcoRI to produce an approximately 1200 base pair fragment containing the majority of the mature mango thioesterase coding sequence. The Ndel/ EcoRI fragment is inserted into Ndel/.EcoRI digested plasmid expression vector pET3a (Νovagen) . For expression analysis, the resulting plasmid is used to transform E. coli strain BL21(DE3) which contains a T7 polymerase (Νovagen) . For expression as a lacZ fusion protein, PCR is conducted with primers 4463 and 4464 to make an -1100 base pair Xbal/EcoRI fragment begining at Xbal site inserted 5' to leucine 112 codon and ending at EcoRI site inserted at nucleotide 1561. Alternatively, primers 4465 and 4464 are used to make an -1200 base pair fragment beginning with Xbal site inserted 5' to proline 81 codon and ending at EcoRI site inserted at nucleotide 1561. PCR conditions are as follows: 94°C 1 min., 60°C 30 sec, 72°C 2 min for 30 cycles. The PCR fragments are cloned into CloneAmp System (GIBCO BRL) . The plasmids are digested with Xbal and EcoRI to produce an approximately 1100 or 1200 base pair fragment containing the majority of the mature mango thioesterase coding sequence from one of the two postulated mature protein N-termini. The Xbal/EcoRI fragment is inserted into Xbal/EcoRI digested plasmid expression vector such as pBCSK (Stratagene) . For expression analysis the vectors are used to transform E coli fadD+ cells (commercially available cells such as SURE cells from BRL may also be used) or an E. coli mutant, fadD, which lacks medium-chain specific acyl-CoA synthetase (Overath et al . , Eur. J. Biochem (1969) 7:559-574). To express the mango M4-23 (pCGN 5234) Class II thioesterase encoding sequence as a lacZ fusion protein, the clone is cut with Sphl and Hindlll. The resulting fragment is isolated from a 0.7% agarose TBE gel using Gene Cleanll Kit (BiolOl) . The fragment is ligated into pUClδ (Novagen; Madison, WI) also digested with Sphl and Hindlll to generate pCGN5235. pCGN5235 is digested with Smal and Hindlll and subcloned into pBC SK (Stratagene) also digested with -Smal and Hindlll to generate pCGN5236. PCR primers 4317 CGTCTAGACATATGCTTGACTGGAAACC 4343 CGGAATTCCGAGACTGCAGTAAGGCTAATC 4463 CUACUACUACUAGCTCTAGAGCTTGACTGGAAACC 4464 CAUCAUCAUCAUCCGAATTCGCAGTAAGGCTAATC 4465 CUACUACUACUAGCTCTAGAGCCAAGGACTTTTAT 4466 CUACUACUACUAGCGCATATGCCAAGGACTTTTAT For thioesterase activity assay a 20 ml culture of E. coli cells containing the mango thioesterase construct and a similar culture of control cells are grown at 25-37°C to an OD600 of -0.5. Induction of the thioesterase expression may be achieved by the addition of IPTG to 0.4mM followed by 1-18 hours further growth. A ten-ml aliquot of each culture is assayed for specific activity towards C10:0-ACP, C12:0-ACP, C14:0-ACP, C16:0-ACP, C18:0-ACP and C18:1-ACP substrates as follows. Cells are harvested by centrifugation, resuspended in 0.4 ml assay buffer and lysed by sonication. Cell debris may be removed by further centrifugation. The supernatant can then used in thioesterase activity assays per Davies et al . . Arch . Biochem & Biophys . (1991) 2-90:37-45 using C10:0-ACP, C12.-0-ACP, C14:0-ACP, C16:0-ACP, C18:0-ACP and C18:1-ACP substrates. For analysis of the fatty acid composition, a 4.5ml sample of E. coli cells grown and induced as described above is transferred into a 15ml glass vial with a teflon- lined cap. lOOμl of a lmg/ l standards solution containing lmg/ml each of Cll:0 free fatty acid, C15:0 free fatty acid, and C17:0 TAG in 1:1 chloroform/methanol is added to the sample, followed by addition of 200μl of glacial acetic acid and 10ml of 1:1 chloroform/methanol. The samples are vortexed to mix thoroughly and centrifuged for 5 minutes at lOOOrpm for complete phase separation. The lower (chloroform) phase is carefully removed and transferred to a clean flask appropriate for use in a rotary evaporator (Rotovap) . The sample is evaporated to near dryness. As medium-chain fatty acids appear to evaporate preferrentially after solvent is removed, it is important to use just enough heat to maintain the vials at room temperature. The dried samples are methanolyzed by adding 1 ml of 5% sulfuric acid in methanol, transferring the samples to a 5ml vial, and incubating the sample in a 90"C water bath for 2 hours. The sample is allowed to cool, after which 1ml of 0.9% NaCl and 300μl of hexane are added. The sample is vortexed to mix thoroughly and centrifuged at lOOOrpm for 5 minutes. The top (hexane) layer is carefully removed and placed in a plastic autosampler vial with a glass cone insert, followed by capping of the vial with a crimp seal. The samples are analyzed by gas-liquid chromatography (GC) using a temperature program to enhance the separation of components having 10 or fewer carbons. The temperature program used provides for a temperature of 140°C for 3 minutes, followed by a temperature increase of 5°C/minute until 230°C is reached, and 230°C is maintained for 11 minutes. Samples are analyzed on a Hewlett-Packard 5890 (Palo Alto, CA) gas chromatograph. Fatty acid content calculations are based on the internal standards. Results of thioesterase activity and fatty acid composition analyses of E. coli cells expressing the mango M4-23 clone are presented in Table II below. TABLE II Acyl-ACP Activity (cpm/μg protein) Substrate M4-23 Control 10:0 26 . 5 29 . 6 12:0 62 . 4 64 . 1 14:0 113 . 8 99 . 8 16:0 757 .4 289 .7 18:0 484 .2 332 . 8 18:1 687 . 0 209 .9 The above results demonstrate that the M4-23 mango Class II thioesterase has specificity towards 16:0-ACP substrates, with some activity also on C18:1-ACP and C18:0- ACP substates. B. Leek PCR reactions are used for insertion of convenient restriction sites into the 5 ' of the mature coding region of the thioesterase cDNA. PCR primers are designed for introduction of unique restriction sites (Sacl-Ndel at 5' end and Kpnl at 3 ' end right after the stop codon) . The entire mature protein encoding region (beginning at the leucine at amino acid position 118 of Figure 4) of the leek thioesterase with the described restriction sites at the ends is generated by PCR. The resulting DNA fragment is inserted into a pBluescript cloning vector to create a LacZ fusion construct, or alternatively ligated into a pET plasmid vector to make a non-fusion construct. E. coli cells transformed with leek thioesterase lacZ fusion constructs are grown and induced by IPTG as described above. Cell lysates are assayed for thioesterase activity as described above. Results of thioesterase activity assays with the 9-1 and 8-2 leek clones indicate that the leek Class II thioesterases have high specificity towards 16:0-ACP substrates, with some minor activities also observed with 14:0, 18:0 and 18:1-ACP substrates. C. Cuphea and Elm Expression of Cuphea hookeriana (CUPH-1 and CUPH-2) and elm Class II acyl-ACP thioesterase clones in E. coli was described in WO 94/10288. The Cuphea CUPH-2 clone demonstrated increased activity with C8 and CIO substrates. This clone was determined to represent the thioesterase activity responsible for the production of C8 and CIO fatty acid in native Cuphea hookeriana seeds. Expression of the CUPH-1 clone, however, revealed increased hydrolysis activity on C16:0 and C14:0-ACP substrates. Since C. hookeriana seeds do not contain significant levels of C14:0 and C16:0 fatty acids, it was not clear if the production of these fatty acids was the direct result of the CUPH-1 expression, or was the result of some activity in the E. coli cells themselves. Total fatty acid analysis of liquid cultures of CUPH-1 transformed K27 ( fadD) are shown in Table III below. Table III Clone Substrate Fattv Acids (nmol/ml) CUPH-1 12:0-ACP 2 12:1-ACP 0.3 14:0-ACP 225.1 14:1-ACP 1.1 16:0-ACP 281 16:1-ACP 124.4 18:1-ACP 92.7 control 12:0-ACP 2.9 12:1-ACP 0.7 14:0-ACP 19.3 14:1-ACP 2.4 16:0-ACP 141.2 16:1-ACP 59.1 18:1-ACP 41.8 Levels of 14:0, 16:0, 16:1 and 18:1 are elevated in comparison to the control. As demonstrated herein by expression of the CUPH-1 clone in transgenic plant cells, the E. coli phenotype is the result of CUPH-1 acyl-ACP thioesterase having activity predominantly on 16:0-ACP substrates, with some lesser activity on 14.-0-ACP. Expression of an elm acyl-ACP thioesterase resulted in increased hydrolysis activity on CIO:0-ACP substrates, which activity could account for the significant levels of C10:0 fatty acids in elm seeds. However, increased activity on C16:0 substrates was also observed, an unexpected result since elm seeds do not contain significant levels of C16:0 fatty acids. Data provided in WO 94/10288 suggested that the C16:0-ACP hydrolysis activity was derived from the E. coli cells, rather than the elm thioesterase. However, as demonstrated herein by expression of elm acyl-ACP thioesterase in seeds of transgenic plants, the observed C16:0 activity in E. coli is due to actual hydrolysis activity of the elm thioesterase on C16:0-ACP. Example 3 Expression of Palmitoyl-ACP Thioesterases in Plants A. Mango A construct for the expression of the mango Class II thioesterase clone in plants under the regulatory control of a transcriptional initiation region from a gene preferentially expressed in plant seed tissue is prepared as follows. A Sail restriction site is introduced upstream of the start codon of the mango thioesterase open reading frame using the mango thioesterase cDNA sequence shown in Figure 2 as a template in a polymerase chain reaction (PCR) . The sense PCR primer (5 '-GCTTGTCGACAAGATGGCTTCTACTG - 3 ' ) includes the mango thioesterase start codon and the -Sail restriction site. The antisense primer (5'- GCGTAAGCTTGCATGCTGGTCA-3 ' ) includes sequence surrounding the unique Sphl site at nucleotide 421 and provides for insertion of a HindiII restriction site downstream of the Sphl site. The product of the PCR reaction (~ 300 bp) is subcloned by digestion with Sail and Hindlll into a chloramphenicol resistant vector pBCSK (Stratagene) that has also been digested with Sail and Hindlll. The mango thioesterase gene is reassembled by combining the -Sphl - -EcoRI fragment from the mango Class II thioesterase encoding sequence (an approximately 1140 bp fragment from the nucleotide 421 Sphl site through the .EcoRI site located 3' to the translation stop codon at nucleotides 1561-1566) with the -300 bp of N-terminal coding sequence using the unique -Sphl and EcoRI sites in the N-terminal fragment/pBCSK construct. The resulting plasmid contains the entire coding sequence and some 3 ' -untranslated sequence of the mango thioesterase gene flanked by Sail and Ncol (nucleotides 1425-1430) restriction sites. A fragment comprising the mango thioesterase sequence is obtained by digestion with Ncol and blunting with the Klenow fragment of DΝA polymerase, followed by digestion with Sail . The blunt end/Sail mango thioesterase fragment is ligated into a napin expression cassette, pCGΝ3223 (described in WO 92/20236) which has been digested with Xhol, blunted and digested with Sail . The resulting expression cassette contains the mango thioesterase flanked by the napin 5 ' - sequences and napin 3 ' -sequences. This expression cassette is cloned into a binary vector for plant transformation by digestion with Asp718 and ligation to Asp718 digested PCGN1557 (McBride et al . (1990) Plant Mol . Biol . 14 : 269- 276) resulting in pCGN5228. Constructs are transformed into Agrobacterium cells, such as of strain EHA101 (Hood et al . , J. Bacteriol (1986) 155:1291-1301), by the method of Holsters et al . (Mol . Gen . Genet . (1978) 153:181-187). B. Leek Clone 9-1 contains the entire transit peptide and mature sequence of the leek Class II thioesterase. Convenient restriction digestion sites are added by PCR and a fragment containing the entire coding region and flanked by a -Sail site at the 5 ' end and and Xhol site at the 3 ' end is isolated. The fragment is ligated into the Sail /Xhol sites of pCGN3223 (napin expression cassette described above). The napin 5 '/leek thioesterase/napin 3' construct is then inserted into pCGN1558, an Agrobacterium/plant transformation binary plasmid (McBride et al . supra) . The resulting construct, pCGN5230, is transformed into an Agrobacterium strain and used to generate transformed plants. C. Cuphea PCR analysis of a Cuphea hookeriana reverse transcribed cDNA (Figure 5 of WO 94/10288) indicated that the 5' region of the TAA 342 CUPH-1 clone was lacking a guanine nucleotide (G) following nucleotide 144 of the sequence shown in Figure 5 of WO 94/10288. [DNA sequence analysis of the CMT9 CUPH-1 clone (Figure 7, herein) confirms the presence of the G nucleotide in that region. Clone CMT-9 can also be used for preparation of CUPH-1 expression constructs, without the requirement for further manipulation of the reading frame in the 5' region.] Thus, a G nucleotide was inserted after nucleotide 144 in TAA 342 by PCR directed mutagenesis resulting in an encoding region beginning at the ATG at 143-145 of the sequence shown in Figure 5 of WO 94/10288. The corrected encoding sequence was cloned into a convenient vector using Sail and Xhol sites (also inserted in the PCR reaction) , resulting in KA2. A Sail fragment of the resulting clone, comprising nucleotides 137-1464 of the sequence shown in Figure 5 of WO 94/10288 (plus the inserted G nucleotide discussed above), was cloned into napin expression cassette pCGN3223. The napin/ Cuphea thioesterase/napin construct was then excised as a Hindlll fragment and cloned into the binary vector pCGN1557 (McBride and Su merfelt (1990) Plant Mol . Biol . 14 : 269-216) . The resulting construct, pCGN4800, was transformed into Agrobacterium tumefaciens and used to prepare transformed plants. C. Elm Acyl-ACP Thioesterase Expression Construct A construct for expression of an elm CIO and C8 acyl- ACP thioesterase in plant seed cells using a napin expression cassette is prepared as follows. The elm ULM-1 medium-chain acyl-ACP thioesterase cDNA does not appear to encode the entire thioesterase transit peptide. Thus, the elm thioesterase coding region was fused to the transit peptide encoding region from the Cuphea CUPH-1 clone as follows. pCGN4800 (CUPH-1 in napin cassette) was digested with Xbal , blunted and digested with StuI to remove the mature protein coding portion of the CUPH-1 construct. The StuI site is located at nucleotides 496-501 of the CUPH-1 sequence shown in Figure 5 of WO 94/10288. The Xbal site is located between the end of the Cuphea thioesterase cDNA sequence and the napin 3' regulatory region. The ULM-1 mature protein encoding region is inserted into the napin /Cuphea transit peptide backbone resulting from removal of the Cuphea mature protein endoding region as follows. The ULM-1 clone is digested with Xbal, blunted and digested with StuI to obtain the elm thioesterase mature protein encoding region. The StuI site is located at nucleotides 250-255 of the sequence shown in Figure 8, and the Xbal site is located at nucleotides 1251-1256, 3' to the stop codon. Ligation of the elm StuI /Xbal fragment into the napin/Cuphea transit peptide backbone results in pCGN4802, having the napin 5' /Cuphea transit:elm mature/napin 3' expression construct. pCGN4803 is transferred to pCGN1557 as a Hindlll fragment resulting in pCGN4803, a binary construct for plant transformation. Example 4 Plant Transformation and Analysis A. Transformation Methods 1. AgrojbacteriuTπ-mediated Transformation Methods which may be used for Agrojbacterium-mediated transformation of Brassica are described by Radke et al . ( Theor. Appl . Genet . (1988) 75:685-694; Plant Cell Reports (1992) 21:499-505) . Transgenic Arabidopsis thaliana plants may be obtained by -AgroJacterium-mediated transformation as described by Valverkens et al . , (Proc. Nat . Acad. Sci . (1988) 55:5536- 5540) . 2. Particle Bombardment DNA sequences of interest may be introduced as expression cassettes, comprising at least a promoter region, a gene of interest, and a termination region, into a plant genome via particle bombardment as described for example in European Patent Application 332 855 and in co- pending application USSN 07/225,332, filed July 27, 1988. Briefly, tungsten or gold particles of a size ranging from 0.5mM-3mM are coated with DNA of an expression cassette. This DNA may be in the form of an aqueous mixture or a dry DNA/particle precipitate. Tissue used as the target for bombardment may be from cotyledonary explants, shoot meristems, immature leaflets, or anthers. The bombardment of the tissue with the DNA-coated particles is carried out using a Biolisticsgb particle gun (Dupont; Wilmington, DE) . The particles are placed in the barrel at variable distances ranging from lcm-14cm from the barrel mouth. The tissue to be bombarded is placed beneath the stopping plate; testing is performed on the tissue at distances up to 20cm. At the moment of discharge, the tissue is protected by a nylon net or a combination of nylon nets with mesh ranging from lOmM to 300mM. Following bombardment, plants may be regenerated following the method of Atreya, et al . , ( Plant Science Letters (1984) 34:379-383) . Briefly, embryo axis tissue or cotyledon segments are placed on MS medium (Murashige and Skoog, Physio . Plant . (1962) 15:473) (MS plus 2.0 mg/1 6- benzyladenine (BA) for the cotyledon segments) and incubated in the dark for 1 week at 25 ± 2°C and are subsequently transferred to continuous cool white fluorescent light (6.8 W/m 2 ) . On the 10th day of culture, the plantlets are transferred to pots containing sterile soil, are kept in the shade for 3-5 days are and finally moved to greenhouse. The putative transgenic shoots are rooted. Integration of exogenous DNA into the plant genome may be confirmed by various methods know to those skilled in the art. B. Plant Analysis Plants transformed with C16:0 acyl-ACP thioesterases as described above are analyzed to determine the percentages of various fatty acids in the seeds. Plants containing elevated levels of C16:0 fatty acids are selected for further analysis, including positional analysis of fatty acids on the TAG molecules. Developing (near-mature) seeds (10-20 seeds/sample) from transgenic plants containing the pCGN4800 and pCGN4803 constructs (expressing CUPH-1 and elm acyl-ACP thioesterases, respectively) are analyzed by GC using parameters as described for analysis of E. coli cell cultures. Results of these analyses from the individual transformant from the pCGN4800 and pCGN4803 transformation events demonstrating the greatest increase in the C16 fatty acid levels are provided in Table IV. Transgenic Brassica napus plants (212/86) expressing the CUPH-1, elm and mango (MANI-2) thioesterase clones are analyzed to determine fatty acid composition. Mature seeds (-20 seeds/sample) from transgenic plants containing the PCGN4800, pCGN4803 and pCGN5288 are analyzed by GC using parameters as described for analysis of E. coli cell cultures. In seeds from pCGN4800 (CUPH-1) transformed plants, increased levels of C16:0 fatty acids are detected. In seeds from pCGN4803 (elm) transformants, significant levels of C16:0 fatty acids are detected along with increases in C14:0 and C10:0 contents. In seeds from pCGN5288 (mango MANI-2) transformed plants, increased levels of C16:0 fatty acids are detected. TABLE IV Fatty Acyl Composition of Transgenic Canola Seeds (mol%) 8:0 10:0 12:0 14:0 16:0 16:1 18:0 18:1 18:2 18:3 20:0 20:1 22:0 Control N.D. N.D. 0.02 0.14 6.45 0.41 1.03 47.32 25.79 17.2 0.39 0.96 0.2 Λ PCGN4800 N.D. N.D. N.D. 0.98 27.61 0.61 2.3 30.13 23.21 12.83 1.03 0.74 0.56 PCGN4803 N.D. 3.94 0.53 5.44 19.96 1.07 2.85 31.36 22.43 11.03 0.64 0.41 0.24 o All publications and patent applications mentioned in this specification are indicative of the level of skill of those skilled in the art to which this invention pertains. All publications and patent applications are herein incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claim.

Claims: What is claimed is: 1. A DNA construct comprising, in the 5' to 3' direction of transcription as operably linked components, a seed-specific promoter regulatory element, a DNA sequence encoding a plant acyl-ACP thioesterase active on C16:0-ACP substrate, and a transcriptional termination region regulatory element functional in a plant cell, wherein said thioesterase sequence element is heterologous to at least one of said regulatory elements. 2. The construct of Claim 1 comprising a leek palmitoyl- ACP thioesterase encoding sequence. 3. The construct of Claim 1 comprising a mango palmitoyl- ACP thioesterase encoding sequence. 4. The construct of Claim 1 wherein said seed-specific promoter element is a napin promoter. 5. The construct of Claim 1 further comprising a T-DNA border element. 6. A plant cell comprising a DNA construct, said DNA construct comprising, in the 5" to 3 ' direction of transcription as operably linked components, a seed- specific promoter regulatory element, a DNA sequence encoding a plant acyl-ACP thioesterase active on C16:0-ACP substrate, and a transcriptional termination region regulatory element functional in a plant cell, wherein at least one element is heterologous to said plant or to another element of said construct. 7. The plant cell of Claim 6 comprising a leek palmitoyl- ACP thioesterase encoding sequence. 8. The plant cell of Claim 6 comprising a mango palmitoyl- ACP thioesterase encoding sequence. 9. The plant cell of Claim 8 wherein said cell is a Brassica cell. 10. The plant cell of Claim 8 wherein said cell is a Brassica napus cell. 11. A plant cell comprising a first DNA construct and a second DNA construct, said first DNA construct comprising, in the 5' to 3 ' direction of transcription as operably linked components, a seed-specific promoter regulatory element, a DNA sequence encoding a plant palmitoyl-ACP thioesterase, and a transcriptional termination region regulatory element functional in a plant cell, and said second DNA construct comprising, in the 5' to 3 ' direction of transcription as operably linked components, a seed-specific promoter regulatory element, a DNA sequence encoding a plant stearoyl-acyl ACP desaturase element positioned in an anti-sense orientation, and a transcriptional termination region regulatory element functional in a plant cell, wherein said plant stearoyl- acyl ACP desaturase is complementary to an indigenous stearoyl-acyl ACP desaturase of said plant cell. 12. A cDNA encoding a plant acyl-ACP thioesterase having hydrolysis on C16:0-ACP substrate. 13. The cDNA of Claim 16, wherein said cDNA sequence is the mango thioesterase sequence shown in Figure 2. 14. The cDNA of Claim 16, wherein said cDNA sequence is the leek thioesterase sequence shown in Figure 4. 15. A method of producing plant seed triglycerides having an increased proportion of C16:0 fatty acyl groups comprising, growing a transgenic plant to produce seed, wherein said plant comprises a construct according to Claim 1. 16. The method of Claim 15 wherein said plant is a Brassica plant. 17. The method of Claim 16 wherein said Brassica plant is a Brassica napus plant. 18. The method of Claim 15 wherein said plant acyl-ACP thioesterase active on C16:0-ACP substrate is from leek, mango, Cuphea hookeriana or elm. 19. A transgenic Brassica napus plant seed comprising a minimum of 8 mole percent palmitate in the storage triglycerides. 20. The seed of Claim 19 comprising a minimum of 10 mole percent palmitate in the storage triglycerides. 21. The seed of Claim 19 comprising a minimum of 20 mole percent palmitate in the storage triglycerides. 22. An oil derived from seed of Claim 19.

Inventors: HAWKINS DEBORAH, JONES AUBREY, KRIDL JEAN, VOELKER TONI ALOIS, YUAN LING, HAWKINS, DEBORAH, JONES, AUBREY, KRIDL, JEAN, VOELKER, TONI, ALOIS, YUAN, LING
Applicants: CALGENE INC, HAWKINS DEBORAH, JONES AUBREY, KRIDL JEAN, VOELKER TONI ALOIS, YUAN LING, CALGENE, INC., HAWKINS, DEBORAH, JONES, AUBREY, KRIDL, JEAN, VOELKER, TONI, ALOIS, YUAN, LING
Application year: 1994
Main classification: C12N15
IPC codes: A01H5, A23D9, C11C3, C12N5, C12N9, C12N15